typedef char *sds;/* Note: sdshdr5 is never used, we just access the flags byte directly.* However is here to document the layout of type 5 SDS strings. */
struct __attribute__ ((__packed__)) sdshdr5 {unsigned char flags; /* 3 lsb of type, and 5 msb of string length */char buf[];
};
struct __attribute__ ((__packed__)) sdshdr8 {uint8_t len; /* used */uint8_t alloc; /* excluding the header and null terminator */unsigned char flags; /* 3 lsb of type, 5 unused bits */char buf[];
};
struct __attribute__ ((__packed__)) sdshdr16 {uint16_t len; /* used */uint16_t alloc; /* excluding the header and null terminator */unsigned char flags; /* 3 lsb of type, 5 unused bits */char buf[];
};
struct __attribute__ ((__packed__)) sdshdr32 {uint32_t len; /* used */uint32_t alloc; /* excluding the header and null terminator */unsigned char flags; /* 3 lsb of type, 5 unused bits */char buf[];
};
struct __attribute__ ((__packed__)) sdshdr64 {uint64_t len; /* used */uint64_t alloc; /* excluding the header and null terminator */unsigned char flags; /* 3 lsb of type, 5 unused bits */char buf[];
};

/* Create a new sds string with the content specified by the 'init' pointer* and 'initlen'.* If NULL is used for 'init' the string is initialized with zero bytes.** The string is always null-termined (all the sds strings are, always) so* even if you create an sds string with:** mystring = sdsnewlen("abc",3);** You can print the string with printf() as there is an implicit \0 at the* end of the string. However the string is binary safe and can contain* \0 characters in the middle, as the length is stored in the sds header. */
sds sdsnewlen(const void *init, size_t initlen) {void *sh;sds s;/* 根据要求的长度返回合适的类型 */char type = sdsReqType(initlen);/* Empty strings are usually created in order to append. Use type 8* since type 5 is not good at this. *//* type 5 不太适合动态扩展，默认用8 */if (type == SDS_TYPE_5 && initlen == 0) type = SDS_TYPE_8;/* 不同的type对应的sdshdr的大小不一样 */int hdrlen = sdsHdrSize(type);unsigned char *fp; /* flags pointer. */sh = s_malloc(hdrlen+initlen+1);/* 如果没有指定初始内容，则将新分配的空间清0 */if (!init)memset(sh, 0, hdrlen+initlen+1);/* 这个判断应该放到前面吧，不然上面memset会导致程序挂掉 */if (sh == NULL) return NULL;/* 找到buf的位置，也是要返回给caller的 */s = (char*)sh+hdrlen;/* 填充对应sdshdr的flags字段 */fp = ((unsigned char*)s)-1;switch(type) {case SDS_TYPE_5: {*fp = type | (initlen << SDS_TYPE_BITS);break;}case SDS_TYPE_8: {SDS_HDR_VAR(8,s);sh->len = initlen;sh->alloc = initlen;*fp = type;break;}case SDS_TYPE_16: {SDS_HDR_VAR(16,s);sh->len = initlen;sh->alloc = initlen;*fp = type;break;}case SDS_TYPE_32: {SDS_HDR_VAR(32,s);sh->len = initlen;sh->alloc = initlen;*fp = type;break;}case SDS_TYPE_64: {SDS_HDR_VAR(64,s);sh->len = initlen;sh->alloc = initlen;*fp = type;break;}}/* 指定了初始内容并且内容非空，填充到新分配的空间当中 */if (initlen && init)memcpy(s, init, initlen);s[initlen] = '\0';return s;
}

/* Enlarge the free space at the end of the sds string so that the caller* is sure that after calling this function can overwrite up to addlen* bytes after the end of the string, plus one more byte for nul term.** Note: this does not change the *length* of the sds string as returned* by sdslen(), but only the free buffer space we have. */
sds sdsMakeRoomFor(sds s, size_t addlen) {void *sh, *newsh;size_t avail = sdsavail(s);size_t len, newlen;char type, oldtype = s[-1] & SDS_TYPE_MASK;int hdrlen;/* Return ASAP if there is enough space left. */if (avail >= addlen) return s;len = sdslen(s);sh = (char*)s-sdsHdrSize(oldtype);newlen = (len+addlen);/* 新长度小于1M就double，否则就增加1M */if (newlen < SDS_MAX_PREALLOC)newlen *= 2;elsenewlen += SDS_MAX_PREALLOC;/* newlen对应的sdshdr的类型 */type = sdsReqType(newlen);/* Don't use type 5: the user is appending to the string and type 5 is* not able to remember empty space, so sdsMakeRoomFor() must be called* at every appending operation. *//* type 5不适合做扩展 */if (type == SDS_TYPE_5) type = SDS_TYPE_8;hdrlen = sdsHdrSize(type);/* 如果前后type不需要变化，则直接使用realloc* 否则，分配新的空间，拷贝原来的内容，释放原来的空间*/if (oldtype==type) {newsh = s_realloc(sh, hdrlen+newlen+1);if (newsh == NULL) return NULL;s = (char*)newsh+hdrlen;} else {/* Since the header size changes, need to move the string forward,* and can't use realloc */newsh = s_malloc(hdrlen+newlen+1);if (newsh == NULL) return NULL;memcpy((char*)newsh+hdrlen, s, len+1);s_free(sh);s = (char*)newsh+hdrlen;s[-1] = type;sdssetlen(s, len);}/* 设置sdshdr的alloc字段(type 5除外) */sdssetalloc(s, newlen);return s;
}